The ancient Egyptians thought that the inundation was caused by the swelling of the river, the result of the falling of a tear of the goddess Isis on a certain night. However, the actual cause was heavy rains upstream. The river would begin to rise in June, be in flood at the end of August, and crest in October.
Before the flood would arrive, men would clear out the main and subsidiary canals and would heap up dykes. Large basins to catch and hold the flood waters were constructed. As soon as they were filled, their openings were blocked off. This water was then available for irrigation after the flooding had receded.